Fishing boats are tied up at a wharf on the waterfront in Portland, Maine. Organizers of a forum about Arctic diplomacy slated to take place in Portland in October say the event will focus on issues like climate change and shipping and it will put a spotlight on its host city. Organizers of a forum about Arctic diplomacy slated to take place in Portland say the event will focus on issues like climate change and shipping and it will put a spotlight on its host city.

The links between some extreme weather events and climate change are becoming easier to measure, say scientists.

A new report from the National Academies of Sciences, Engineering, and Medicine (NAS) is “the first definitive ranking of what events can be attributed to climate change," said University of Georgia scientist J. Marshall Shepherd, one of the committee members who contributed to the report.

The highly regarded NAS "brings a gold standard to the assessment of the science," he added.

"While that question remains difficult to answer given all the factors that affect an individual weather event,” said Dr. David Titley, the head of the committee that wrote the report, in a statement, “we can now say more about how climate change has affected the intensity or likelihood of some events."

He later said, “Our reports say that you can do this for some events now, especially heat/cold (higher confidence) and heavy rainfall/drought (medium confidence)." The link between a higher global temperatures and a particular heat wave is easy to understand. The connection between global climate change and hurricanes or cyclones is harder to trace, he added, because so many different factors contribute to dramatic storms.

“All weather events result from a combination of both natural and human-influenced factors," says the report brief, "that together produce the specific conditions for a particular event.”

Scientists who study what they call "event attribution" investigate the extent to which climate change influenced the probability or intensity of a given event.

The report authors agree with the near-universal scientific consensus that human fossil fuel use and greenhouse gas emissions have led to melting ice sheets and warmer global temperatures, and they add that it is important to understand the full implications of climate change on global weather patterns through the study of event attribution.

Climatologists' growing ability to specify the links between climate change and weather events can be attributed to technology and the length of human climatic records. Computer modeling allows scientists to simulate events under atmospheric conditions with human greenhouse gas emissions and without them, and therefore determine the how often the extreme weather event would occur with or without anthropogenic climate change.

“We caution against extrapolating from one study to make big sweeping statements about all aspects of climate change,” said Dr. Shepherd. “We also caution that there is some selection bias in what events are studied.”

The study of event attribution is fairly new. According to the preface of the 144 page report, the first attempt at event attribution occurred in 2004, when scientists studied the 2003 European heat wave that killed tens of thousands of people. Most growth in the field has come since 2012.

The report repeatedly emphasizes that questions like "Did climate change cause superstorm Sandy and hurricane Katrina?" are almost impossible to answer definitively, since natural variability always plays a role in any particular weather event.

They encourage questions like "Are events of this severity becoming more or less likely because of climate change?" or "To what extent was the storm intensified or weakened, or its precipitation increased or decreased, because of climate change?"